Manufacturing method of martensite stainless seamless steel pipe

ABSTRACT

The present invention relates to a manufacturing method of martensite stainless seamless steel pipe for oil well pipes and couplings. The martensite stainless seamless steel pipe has the chemical components comprising, by mass percent, 0.15 to 0.22% of C, 0.18 to 1.0% of Si, 0.05 to 1.0% of Mn, 10.5 to 14.0% of Cr, and the balance of Fe and impurities that include, by mass percent, not greater than 0.20% of Ni, not greater than 0.05% of Al, not greater than 0.100% of N, not greater than 0.015% of S, and not greater than 0.020% of P. The method comprises the steps of piercing and rolling a billet having the said chemical components to obtain a seamless steel pipe having a cross-sectional area being 10 to 95% of that of said billet; then heating the seamless steel pipe at a temperature of 850 to 980° C., and then subjecting said seamless steel pipe to quenching and tempering in the condition that let P value in the follow expression be 18500 to 22200. 
 
 P=T ·(17.7+log  t ) 
wherein “T” represents soaking temperature (K), “t” represents soaking time (second), “log t” represents the value of common logarithm of “t”. The martensite stainless seamless steel pipe with high weatherability can be obtained according to the present invention.

TECHNICAL FIELD

The present invention relates to a manufacturing method of martensite stainless seamless steel pipe, in particularly, a manufacturing method of martensite stainless seamless steel pipe for oil well pipes and couplings.

TECHNICAL BACKGROUND

Stainless steel containing about 12 to 15% Cr is usually called “13% Cr steel”. It is used as materials for oil well pipes and others due to its excellent performances in heat resistance and mechanical characters including proof stress and yield stress.

DISCLOSURE OF THE INVENTION

In order to obtain 13% Cr steel with excellent weatherability, various researches in respect to its manufacturing methods have been carried out.

An object of the present invention is to provide a manufacturing method of martensite stainless seamless steel pipes with excellent weatherability.

Specifically, the method according to the present invention is a method for manufacturing martensite stainless seamless steel pipe which has the chemical components including, by mass percent, 0.15 to 0.22% of C, 0.18 to 1.0% of Si, 0.05 to 1.0% of Mn, 10.5 to 14.0% of Cr, and the balance being Fe and impurities that include, by mass percent, not greater than 0.20% of Ni, not greater than 0.05% of Al, not greater than 0.100% of N, not greater than 0.015% of S, and not greater than 0.020% of P. The manufacturing method comprises the steps of piercing and rolling a billet having the above-stated chemical components to obtain a seamless steel pipe having a cross-sectional area being 10 to 95% of that of billet; then heating the seamless steel pipe at a temperature of 850 to 980° C., and then subjecting the same to quenching and tempering in the condition that let P value in the follow expression be 18500 to 22200. P=T·(17.7+log t) wherein “T” represents soaking temperature (K), “t” represents soaking time (second), “log t” represents the value of common logarithm of “t”.

The martensite stainless seamless steel pipe made according to this method may be used for couplings of oil well pipes, with the P being 18500 to 21500.

BEST MODE OF THE INVENTION

The present invention is described in the aspects of (1) the chemical components of steel (base metal), (2) processing conditions, and (3) quenching and tempering process, as follows.

(1) The chemical components of the steel (base metal) include indispensable components such as C, Si, Mn and Cr and may also include impurities such as Ni or the like. The chemical components of the steel and the reasons for the limitation thereof are described as follows. Here, “%” related to the chemical components represents “mass %”.

C: 0.15 to 0.22%.

C is an element for improving the strength. In order to ensure the strength not less than the grade of 80 ksi (551.58 MPa), the C content must be controlled at 0.15% or more. However, the excessive C content would degrade the corrosion resistance. Therefore, the C content must be controlled at not greater than 0.22%.

Si: 0.18 to 1.0%.

Si is an indispensable element acting as a deoxidizer for decreasing oxygen which causes deterioration of hot workability during the steel-manufacturing stage. Furthermore, Si can restrain the forming of scales, and improve the adhesiveness. In order to obtain those effects, the Si content must be controlled at 0.18% or more. However, the excessive Si content would degrade the toughness. Therefore the Si content must be controlled at not greater than 1.0%.

Mn: 0.05 to 1.0%.

Similar to Si, Mn is an indispensable element acting as a deoxidizer. Furthermore, Mn has the effect of improving the hot workability by fixing the solid-dissolving S in the steel in the form of MnS. In order to obtain such effect, the Mn content must be controlled at not less than 0.05%. However, the excessive Mn content would degrade the toughness, and the inner layer scale would become fragile and easy to desquamate due to the formation of the spinel oxide of FeO.Mn₂O₃. Therefore, the Mn content must be controlled at not greater than 1.0%. If further improvement of toughness is required, the Mn content is preferably controlled at not greater than 0.85%.

Cr: 10.5 to 14.0%.

Cr is an element for improving the corrosion resistance, and particularly for the excellent corrosion resistance against CO₂. In order to prevent the emergence of pitting corrosion and cracking corrosion, the Cr content must be controlled at not less than 10.5%. On the one hand, Cr is an element to form ferrite. When the Cr content exceeds 14.0%, the formation of δ (delta) ferrite in high temperature heating condition would degrade the hot workability. Simultaneously, due to the increasing existence of ferrite, predetermined strength cannot be obtained even if tempering is carried out for avoiding the impairment of stress corrosion-cracking resistance. Therefore, the Cr content must be controlled at not greater than 14.0%.

Ni: not greater than 0.20%.

Ni is an element causing the hydrogen sulfide stress cracking. Therefore, the Ni content must be controlled at not greater than 0.20%. On the other hand, Ni has the effect of improving the adhesiveness between the base metal and inner layer scale. Therefore, the Ni content is preferably controlled at not less than 0.02%.

Al: not greater than 0.05%.

Al is an element causing the impairment of steel cleanliness. Furthermore, it is the cause of nozzle blockage during the continuous casting of base metal. Therefore, the Al content must be controlled at not greater than 0.05%. On the other hand, Al is an effective deoxidizer. Therefore, the Al content is preferably controlled at not less than 0.0005%.

N: not greater than 0.100%.

N is an element causing the decrease of toughness due to its excessively high content. Therefore, the N content must be controlled at not greater than 0.100%. On the other hand, N can improve the strength by intensifying the solid solution. Therefore, the N content is preferably controlled at not less than 0.010%.

S: not greater than 0.015%.

S exists in steel as an impurity. The large content of S would significantly decrease the hot workability. This influence would become remarkable in, for example, the rolling process by a plug mill or a mandrel mill, or in the piercing process by a piercer. Thus, the steel pipe without defects cannot be produced due to the excessive large content of S. Therefore, the S content must be controlled at not greater than 0.015%.

P: not greater than 0.020%.

Similar to S, P exists in steel as an impurity. When the content of P is too large, the generation of flaws in the steel cannot be prevented, and the toughness would be significantly decreased. Therefore, the P content is preferably controlled at not greater than 0.020%.

(2) The manufacturing method of martensite stainless seamless steel pipe of the present invention comprises the following steps (A) and (B):

(A) Billet Preparation and Pipe Preparation

First, a billet containing the above-described chemical components is prepared; then the billet is heated and subjected to piercing and rolling with a Mannesman mill or a plug mill so as to obtain a raw pipe; and then the raw pipe is subjected to size rolling by a mandrel mill or a stretch reducer so as to obtain a martensite seamless steel pipe having a predetermined outer diameter and a predetermined wall thickness. The cross section area of the seamless steel pipe obtained in this step is 10% to 95% of that of the billet.

(B) Quenching and Tempering of Seamless Steel Pipe Obtained in Step (A)

(a) The base metal is heated in the quenching furnace before quenching and tempering. The atmosphere in the quenching furnace may be the gas atmosphere of LNG, LOG, heavy oil, butane or the like. Specifically, the atmosphere in the quenching furnace is preferably composed of 0.01 to 8.0% of O₂, 3.0 to 20.0% of H₂O, 1.0 to 20.0% of CO₂, and balance contents of N₂ and others. Otherwise, the inadequate combustion or the surplus O₂ would increase the unit requirement, or require the blowing-in of additional gas and steam, thus increasing the cost.

The base metal is heated to 850 to 980° C. in the quenching furnace. Here, if the heating temperature is lower that 850° C., it is impossible to make the base metal become austenite monophase, and simultaneously the toughness of seamless steel pipe would be decreased. Furthermore, if the heating temperature is lower than 850° C., it is difficult to remove the scale layer.

After above heating process, the steel is subjected to quenching by, for example, air-cooling the base metal so as to transform the base metal into the martensite structure.

(b) In the tempering process, the atmosphere in the tempering furnace may be similar to that of the quenching process. The composition of the atmosphere is similar to that described above.

The condition of soaking time and soaking temperature of the seamless steel pipe in the tempering furnace is defined to let P value in the following expression be 18500 to 22200. P=T·(17.7+log t) wherein “T” represents the soaking temperature (K), “t” represents the soaking time (second), “log t” represents the value of common logarithm of “t”.

After soaking for the predetermined time under the predetermined temperature, the seamless steel pipe is tempered, for example, by air cooling.

The martensite stainless seamless steel pipe with high weatherability can be obtained by this manufacturing method.

Furthermore, in order to obtain the seamless steel pipe for couplings used to connect oil well pipes by screwed nipple, the condition of soaking temperature and soaking time are defined to let P value be 18500 to 21500.

EFFECTS OF THE INVENTION

The martensite stainless seamless steel pipe with high weatherability can be obtained according to the present invention. 

1. A manufacturing method of martensite stainless seamless steel pipe with chemical components comprising, by mass percent, 0.15 to 0.22% of C, 0.18 to 1.0% of Si, 0.05 to 1.0% of Mn, 10.5 to 14.0% of Cr, and the balance of Fe and impurities that include, by mass percent, not greater than 0.20% of Ni, not greater than 0.05% of Al, not greater than 0.100% of N, not greater than 0.015% of S, and not greater than 0.020% of P; said method comprising the steps of piercing and rolling a billet having the said chemical components to obtain a seamless steel pipe having a cross-sectional area being 10 to 95% of that of said billet; then heating the seamless steel pipe at a temperature of 850 to 980° C., and then subjecting said seamless steel pipe to quenching and tempering in the condition that let P value in the follow expression be 18500 to
 22200. P=T·(17.7+log t) wherein “T” represents soaking temperature (K), “t” represents soaking time (second), “log t” represents the value of common logarithm of “t”.
 2. A manufacturing method of martensite stainless seamless steel pipe according to claim 1, wherein said P value is 18500 to 215000, and said martensite stainless seamless steel pipe obtained thereby is used for couplings of oil well pipes. 